The present invention relates to determination of an optical property of a device under test (DUT), e.g. an optical fiber, by means of Optical Time Domain Reflectometry (OTDR) measurement.
OTDR and respective devices are common and indispensable tools for optical fiber network installers to perform optical characteristics measurements. Installation and commissioning of optical fiber links in optical fiber networks normally require the test of the fiber for loss. However, not only link- and splice-loss, but most often (especially in video and high data rate optical fiber systems) also return-loss and reflectance of the optical fiber. The latter is a characteristic figure for reflection that generally can be determined by an OTDR only if the backscatter factor of the fiber under test is known.
An OTDR usually probes the fiber under test with a short laser pulse and displays the fiber's response as a power vs. distance graph. A detailed technical description about the state of the art of OTDR measurements is given by Dennis Derickson in “Fiber Optic Test and Measurement Handbook, Chapter 11, by Prentice-Hall Inc., Upper Saddle River, N.J. 07458, USA, 1998”.
As splice-loss and fiber attenuation measurements are relative power measurements, no further information about the fiber under test is needed. Only for accurate distance measurements the exact refractive index of the optical fiber under test must be known. On the other hand, the determination of the strength of a reflection requires the correct backscatter factor of the fiber. However, this figure is rarely known, because it varies with wavelength of the probing signal and with fiber type. Therefore, in prior art OTDRs typical default values are used as replacements for the real backscatter factor, which makes such OTDR based determinations of the strength of a reflection not very reliable.
Even with sophisticated laboratory-type test equipment it is not straightforward to determine the backscatter factor of an optical fiber. Besides that, also in a laboratory, a very careful and methodical working style is mandatory for accurate results. Such a working style can not be performed outside the laboratory. To date there is no proven and reliable technique for measuring the backscatter factor outside the laboratory, e.g. in the field.